In the context of such fans, a desire exists for them to run at full output, i.e. at a rotation speed of, for example, 4000 rpm, only when the temperature of the object to be cooled is high. It is possible for this purpose, by means of a sensor on or in said object, to generate a temperature signal, and with that a PWM (Pulse Width Modulated) signal whose duty factor depends on the temperature of said object, so that, for example, at a temperature of 20° C., the duty factor is low and the fan consequently runs slowly, since little heat needs to be removed. If, in contrast, the temperature of the object is 70° C., the duty factor is then increased to, for example, 80% and the fan runs correspondingly faster, so that the larger quantity of heat can reliably be dissipated.
The result of this is that such fans have a longer service life, and at low temperatures a fan of this kind is almost or entirely inaudible, since it is running slowly.
With fans of this kind, which must be very inexpensive, the problem arises of mapping the temperature information contained in a PWM signal of the kind described onto the rotation speed of the motor or fan as completely as possible and in the manner of a conformal mapping; in other words, as little as possible of this temperature information should be lost.
For example, it may be that a rotation speed of 15% of the maximum rotation speed of the fan should correspond to a duty factor of 15%, a rotation speed of 20% of maximum speed to a duty factor of 20%, etc.
If, as a result of peculiarities of the circuit and the design of the motor, the latter works in such a way that the rotation speed consistently has a value of 15% of the maximum rotation speed in a duty factor range from 15 to 50%, and rises only above a duty factor of 50%, the temperature information in the duty factor range from 15% to 50% is therefore lost. This is undesirable, because conformal mapping is not taking place, and because the risk exists that the object to be cooled will overheat.